Inverted tooth chains have long been used to transmit power and motion between shafts in automotive applications and they are conventionally constructed as endless chains with ranks or rows of interleaved inside links or link plates each having a pair of toes, and each having two apertures that are aligned across a link row to receive pivot pins (e.g., straight pins, rocker joints, etc.) to join the rows pivotally and to provide articulation of the chain as it drivingly engages the sprocket teeth either at the inside flanks (“inside flank engagement”) or at the outside flanks (“outside flank engagement”) of the link plate toes at the onset of meshing with the driving and driven sprockets. Although both meshing styles have been used for automotive timing drives, inside flank engagement is more commonly used for these drives. Guide link plates are located on opposite sides of alternating rows of inside link plates in order to position the chain laterally on the sprockets.
Chain-sprocket impact at the onset of meshing is the dominant noise source associated with chain drive systems and it occurs as the chain links leave the span and collide with a sprocket tooth at engagement. The complex dynamic behavior of the meshing phenomenon is well known in the art and the magnitude of the chain-sprocket meshing impact is influenced by various factors, of which polygonal effect (referred to as “chordal action” or “chordal rise”) is known to induce a transverse vibration in the “free” or unsupported chain span located upstream from the sprocket as the chain approaches the sprocket along a tangent line. This chordal action occurs as the chain enters the sprocket from the taut span during meshing and it will induce chain motion in a direction perpendicular to the chain travel but in the same plane as the chain and sprockets. It is known that chordal action and the resulting inherent undesirable oscillatory chain motion will add to the severity of the chain-sprocket meshing impact and further contribute to the related chain engagement noise levels.
FIGS. 1A and 1B serve to illustrate the chordal rise for a sprocket in which chordal rise CR is conventionally defined as the displacement of a chain pin center C (i.e., axis of articulation of a pin, rocker joint etc.) as it moves through an angle α/2, where:CR=rp−rc=rp[1−cos(180°/N)]and where rc is the chordal radius or the distance from the sprocket center to a sprocket pitch chord of length P; rp is the theoretical pitch radius of the sprocket, i.e., one-half of the pitch diameter PD; N is the number of sprocket teeth; and α is equal to the sprocket tooth angle or 360°/N. FIG. 1A shows the chain pin center C at a first position where it has just meshed with the sprocket and where its center is simultaneously aligned with both the tangent line TL along which the chain approaches the sprocket for meshing and the sprocket pitch diameter PD. FIG. 1B illustrates the location of the same pin center C after the sprocket has rotated through the angle α/2, where it can be seen that the pin center C must be transversely displaced in order to continue its travel around the sprocket wrap, and this transverse displacement of the pin center results in a corresponding displacement of the upstream chain span and tangent line TL thereof. This repeated transverse displacement of the chain pin centers C as they move through the chordal rise serves to induce undesired vibration in the approaching unsupported chain span located upstream from the sprocket which increases meshing noise.
One attempt to eliminate undesired chordal action of the chain is described in U.S. Pat. No. 6,533,691 to Horie et al. Horie et al. disclose an inverted tooth chain wherein the inside flanks of each link plate are defined with a compound radius profile intended to smooth the movement of the inside flanks from initial contact with the sprocket to the fully meshed (chordal) position. The chain disclosed by Horie et al. is also intended to lift the chain intentionally a distance “h” (see FIG. 7 of Horie et al.) above the tangent line in an effort to tension the slack side of the system to eliminate vibration in the slack strand of the chain.
Chain lift is also intentionally increased in the system disclosed in published U.S. patent application no. 2004/0110591 by Kotera. There, the prominence of the inside flanks of the chain relative to the respective outside flanks of adjacent link plates is defined as a function of the chain pitch P. In particular, the maximum projection of the inside flank δmax relative to the related outside flank is said to fall in the range of 0.021×P≦δmax≦0.063×P and most preferably in the range of 0.035×P≦δmax≦0.063×P in an effort to restrain transverse vibration of the chain by lifting the chain above the tangent line. By way of example, according to the Kotera application, the maximum projection of the inside flank δmax relative to the related outside flank for a 7.7 millimeter (mm) chain pitch P will be in the range of 0.162 mm ≦δmax≦0.485 mm.
It is believed that these conventional approaches to minimize vibration resulting from chordal action do not optimize chain/sprocket meshing dynamics, and may, in fact, be detrimental. In an effort to compensate for the chordal action of the chain and sprocket by intentionally introducing chain lift, the chain is nevertheless forced out of a straight line path. Thus, it has been deemed desirable to provide an inverted tooth chain and sprocket system with inside flank engagement that affords minimal perpendicular chain movement in the span to minimize transverse vibration in the unsupported chain span during the meshing process.